Zinc oxide nanoflakes for treatment of pollutants

ABSTRACT

A method for preparing leaf like ZnO nanoflakes by anodization of zinc foil in a mixture of ammonium sulfate and sodium hydroxide electrolytes is described.

FIELD OF INVENTION

Metal oxide semiconductors have been widely studied and have received considerable attention in recent years due to their optical and electrical properties [1]. Rapid development in the field of wide band gap semiconducting oxides and low-dimensional semiconductor structures has stimulated intensive research efforts due to their importance in fundamental scientific studies and potential applications. Wide band gap semi-conducting oxides have been perceived as an attractive material for renewable energy devices and this wide band-gap semiconductor can be used in optoelectronic and electronic devices, as well as in the field of electrochemistry for the production of chemical sensors and solar cells.

BACKGROUND OF INVENTION

Among one-dimensional semiconductor materials and wide band gap semiconducting oxides (ZnO) nanostructures have attracted the most intensive research attention in recent years due to its unique properties of near-UV emission, good optical transparency, electric conductivity, excellent thermal and chemical stability, low growth temperatures and good potential for scale-up. Due to its unique properties, ZnO nanomaterials have been considered to have great potential applications in ultraviolet lasers, solar cells, gas sensors, luminescent, bio-detectors, UV light-emitting diodes, photo catalysts, field emitter, transparent conductors, electronic, and optical display devices.

The properties of ZnO are strongly dependent on its structures, which include the morphology, size, orientation, and density of the crystal. Developments of a controllable synthesis of ZnO nanomaterials having specific morphology are important to explore their potential applications as smart and functional materials. Until now, a variety of ZnO nanostructures such as nanowires, nanorods, nanotubes, nanobelts, and nanoflowers have been synthesized using various techniques such as sol-gel method, anodization method, sonochemical method, chemical bath deposition, hydrothermal synthesis, gas phase process and chemical vapor deposition.

Anodization technique is the most versatile surface treatments, providing a metal with oxide films that allow enhancement of corrosion/wear resistance and decoration of a metal surface with diverse colors. It is a very cost-effective method to produce very uniform and adhesive oxide films on metals. However, reports on the formation of nanostructured zinc or zinc oxide by anodization are rarely found since zinc oxide is very easily dissolved in both acidic and highly alkaline conditions, and it will also dissolve in neutral solution under light illumination. The preparation of ZnO nanoparticles and nanowires by anodization method have been attempted by using a NaOH based electrolyte and a mixture of HF and ethanol. It was also reported that single-crystalline Zn microtips can be obtained by anodization in a mixture of NH₄Cl and H₂O₂.

Meanwhile, photocatalytic degradation of organic pollutants in water and air using semiconductive particles, such as TiO₂ and ZnO, has attracted extensive attention in the past two decades due to their high photocatalytic activity, non-toxic nature, low cost, excellent chemical and mechanical stability. Although TiO₂, is universally considered as the most active photocatalyst, ZnO can also be a suitable alternative to TiO₂ because it is lower in cost and has a similar band gap energy (around 3.2 eV).

Nowadays, environmental problems associated with hazardous wastes and toxic water pollutants have attracted much attention. Wastewater from textile, paper and some other industrial processes contain residual dyes, which are non-biodegradable. Releasing toxic and potential carcinogenic substances such as dyes into the aqueous phase create severe environmental pollution problems. Various chemical and physical processes such as coagulation/flocculation, activated carbon adsorption, reverse osmosis and ultrafiltration techniques have been developed in order to remove the color from textile effluents. However, these techniques are non-destructive, since they only transfer the non-biodegradable matter into sludge, giving rise to new type of pollution, which needs further treatment. Hence, attention has to be focused on techniques that lead to the complete destruction of the dye molecules.

SUMMARY OF THE INVENTION

Herein, we present the invention of anodization of zinc in mixture of ammonium sulfate and sodium hydroxide solution at room temperature. In addition, the effect of different conditions of the anodization process particularly the applied voltage, concentration of sodium hydroxide and stirring effect on the deposition morphology and composition were optimized. Catalytic activities of zinc oxide were investigated by measuring the photocatalytic degradation of methylene blue (is a potent cationic dye) in aqueous solution under the illumination of UV light.

The chemical reagents used were ammonium sulfate, (NH₄)₂SO₄ (Merck), sodium hydroxide, NaOH (R&M Chemicals), 99.0% zinc foil (R&M Chemicals), 99.95% platinum wire (Street Chemicals), acetone, C₃H₆O (HmbG) and methylene blue (R&M Chemicals). All chemicals were used as received without any further purification. Ultrapure water is produced by SG-Ultraclear [0.05 μS/cm].

The high purity (99.0%) zinc foils with 0.2 mm thickness were used in this study. Prior to surface treatment, the foils were degreased by sonication in acetone for 15 min.

The foils were then rinsed with ultrapure water, and dried in nitrogen stream. Electrochemical experiments were performed using a direct current voltage source (Zhao Xin RXN-303D). Zinc foils (surface area=6.0 cm²) was used as anodic electrode while platinum wire (99.95%), was used as cathodic electrode. The distance between anodic and cathodic electrodes was 3.0 cm. The electrolyte used in this process was a mixture of (NH₄)₂SO₄ and NaOH. Electrolyte solutions with varying concentrations were prepared from reagent grade chemicals and ultrapure water. All anodic deposition experiments were performed at room temperature. After anodizing for a specific duration, the foil was immediately removed from the solution and washed with ultrapure water. Finally, it was dried in oven overnight prior to characterizations.

The morphology and composition of the prepared ZnO were characterized by field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) spectroscopy (Leo Supra 50VP SEM). Powder X-ray diffraction (XRD) analysis was performed using an X-ray diffractometer (Siemens D500 Kristallofex) with Cu Ku radiation (λ=1.5418 Å).

Photocatalytic activity of the zinc oxide was evaluated by measuring the photocatalytic degradation of methylene blue in water under the illumination of UV-light. The initial concentration of methylene blue solution used was 10 mg/L. Reaction suspensions was prepared by adding catalyst (Zn foil of 3 cm²) to a 50 mL methylene blue solution. Prior to irradiation, the aqueous solution was stirred continuously in the dark for 30 min to ensure adsorption/desorption equilibrium. The photocatalytic reaction was conducted at room temperature under UV light from a UV tube positioned parallel to the beaker at a distance of 15 cm and the reaction timing was started. Each experiment was conducted for 1 h with 5 mL sample, which was extracted every 15 min and filtered. The decomposition of MB was monitored by measuring the absorbance of the aliquot solution using the UV-Vis Spectrophotometer (US-HIO Optical Modulex) at 665 nm. Blank experiment was conducted in the absence of the catalyst with irradiation.

DESCRIPTION OF THE DRAWINGS

FIG. 1: EDX spectra of ZnO produced in (a) a mixture of (NH₄)₂SO₄ and NaOH solution, (b)NaOH solution.

FIG. 2: XRD diffractograms of ZnO produced in (a) a mixture of (NH₄)₂SO₄ and NaOH solution, (b) NaOH solution.

FIG. 3: SEM images of zinc foil anodized in 0.050 M (NH₄)₂SO₄ and NaOH electrolyte at room temperature for 90 min at 10 V with different concentration of NaOH: a) 0.0125 M, b) 0.0250 M, c) 0.0375 M

FIG. 4: SEM images of zinc foil anodized in 0.050 M (NH₄)₂SO₄ and 0.025 M NaOH electrolyte at room temperature for 90 min at different voltage: a) 5 V. b) 10 V. c) 15 V

FIG. 5: SEM images of zinc foil anodized in 0.050 M (NH₄)₂SO₄ and 0.025 M NaOH electrolyte at room temperature for 90 min at 10 V: a) without stirring, b) with stirring

FIG. 6: Photodegradation of methylene blue using catalyst produced using anodization method at room temperature at 10 V for 90 min with 0.050 M (NH₄)₂SO₄ and various concentration of sodium hydroxide.

FIG. 7: Photodegradation of methylene blue using catalyst produced using anodization method in a mixture of 0.050 M (NH₄)₂SO₄ and 0.0250 M NaOH at room temperature for 90 min at different voltage.

FIG. 8: Photodegradation of methylene blue using catalyst produced using anodization method in different electrolytes at room temperature at 10 V for 90 min.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 a shows the energy dispersive X-ray (EDX) analysis spectrum of the ZnO produced in a mixture of (NH₄)₂SO₄ and NaOH solution. The results indicated that besides zinc and oxygen, sulfur was also present in the product obtained. High percentage of atomic oxygen maybe explained by the existence of impurity such as SO₄ ²⁻ ions. The presence of carbon in all samples is due to the double-sided tape used to attach the sample during the scanning process. Anodization in the presence of NaOH alone results in the formation of pure ZnO (FIG. 1 b) as the atomic ratio of zinc to oxygen was approximately 1:1 and this is also consistent with the XRD analysis. The EDX analysis was conducted to determine the elemental composition of the product obtained.

FIG. 2 shows the XRD patterns for ZnO produced in a mixture of ammonium sulfate and sodium hydroxide solution and ZnO produced in sodium hydroxide alone. Two sets of peaks, which are readily indexed to the hexagonal ZnO phase (JCPDS 36-1451) and pure zinc, which clearly originated from zinc foil (JCPDS 65-3358) [8]. These XRD results showed that ZnO can be synthesized using the present anodizing condition. We noticed that peak due to SO₄ ²⁻ ions was absence in the diffractogram of the product obtained using the mixture of (NH₄)₂SO₄ and NaOH solution (FIG. 1 a). The absence could be related to the amorphous characteristic of the SO₄ ²⁻ impurities present on the foil.

The SEM analysis was performed to study the surface morphology of the ZnO produced in different sodium hydroxide concentrations and the images are shown in FIG. 3. The figure shows that at the sodium hydroxide concentrations of 0.0125 M (FIGS. 3 a) and 0.0250 M (FIG. 3 b), ZnO exhibited a non-directional leaf-like nanoflakes. However, at the concentration of 0.0375 M, it did not exhibit the same morphology.

When the concentration was increased from 0.0125 to 0.0250 M, the amount of nanoflakes formed increase, while the size of the flakes decreased and the vacancies between flakes became smaller. Further increase in the concentration of sodium hydroxide leads to the formation of different ZnO morphology as shown in FIG. 3 c. Based on the proposed mechanism of ZnO deposition [27], we believe that the morphology of ZnO will strongly depend on the concentration of the electrolyte. The varying morphology of the nanostructures formed is not well understood. However, it is speculated that the structure must be related to the degree of supersaturation of ZnO nuclei on the foil surface [3]. Abd El Rehim et al. [27] also reported the same behavior for ZnO produced in 0.1 M NaOH containing various concentration of Na₂SO₄, Na₂SO₃, Na₂S, Na₂S₂O₃ or NH₄SCN. Abd El Aal [28] has reported the anodic behavior of Zn foil produced using 0.001 M Na₂B₄O₇ solution with and without the presence of various concentration of Na₂SO₄, Na₂S or Na₂S₂O₃. Both literatures stated that the presence of SO₄ ²⁻ anions stimulates the active dissolution of Zn. At low concentrations of hydroxide ions, there are only few hydroxide ions that can react with Zn²⁺ ions to form zincate ions, Zn(OH)₄ ²⁻ which finally form ZnO. Hence, in the initial stage, the ZnO growth will be slowed and less ZnO was obtained.

FIG. 4 shows the effects of applied potentials on the formation of ZnO. The ZnO obtained exhibit a non-directional leaf-like nanoflakes. As the applied potentials increase from 5 to 10 V, the size of ZnO flakes became visibly larger. At higher potential, 15 V, the size of ZnO remain almost the same. This can be explained by the fact that the heating produced by the anodic current leads to a rise in the local temperature. Consequently, the chemical etching is accelerated under higher anodic voltage, which counteracts the effect of an increasing ZnO layer. As a result, the size of ZnO would not be proportional to the applied anodic voltage [2]. Kim et al. [16] have also reported the effect of applied potentials on the formation of ZnO stripes through anodization in a mixture of ethanol and sulfuric acid solution. The authors reported that, as the applied potential increases, the gap between neighboring stripes increase. However, this phenomenon cannot be observed in this experiment.

The ZnO formed under stirred condition were more uniform and the size of nanoflakes were almost the same (FIG. 5) compared to its counterpart. Under stirring condition, equilibrium was more easily reached and the ions were equally distributed inside the electrolyte.

The photocatalytic activity of the ZnO prepared under different anodic conditions was evaluated for the photodegradation of methylene blue (MB). Degradation rate was calculated using the equation below:⁸

Degradation rate=(1−c/c _(eq))×100%

Results are shown in FIG. 6. During the first 15 min, only 0.24% of methylene blue has been degraded. After 1 hour, 1.1% of the dye was degraded. The results show that photolysis of MB without catalyst still takes place but only to a small extent.

According to Jang et al. and Shen et al., since the photocatalytic degradation occurs predominantly on the surface of the photocatalyst, studies on the adsorption of the dyes from aqueous solution onto ZnO surface are relevant and important. The equilibrium concentration of the dye (c_(eq)) in contact with the catalyst, instead of that of the feed dye solution, represents the true dye concentration in the solution at the start of irradiation.

FIG. 6 shows the photodegradation of MB using catalyst produced via anodization process under room temperature at 10 V for 90 min in a mixture of 0.050 M (NH₄)₂SO₄ with various concentration of sodium hydroxide. When the concentration of sodium hydroxide increases, the decomposition ratio increases with the degradation time. The first 15 min showed that at low to high NaOH concentration, the percentage of degradation of MB increased only slightly from 0.31% to 1.16%. After 60 min, 1.86% and 2.16% of dye was degraded in the NaOH concentration of 0.0125 M and 0.0375 M respectively. Increasing in concentration of sodium hydroxide increases the formation of zinc oxide nanoflakes, which showed the efficiency of degradation of MB. After 60 min, the percentage of degradation of MB by the photocatalyst prepared using 10 times more concentrated NaOH was 9.57% where as the catalyst prepared using 0.0250 M sodium hydroxide alone showed a 2.10% degradation.

Photodegradation. of MB using catalyst produced in a mixture of 0.050 M (NH₄)₂SO₄ and 0.0250 M NaOH at room temperature for 90 min anodization time at different voltage were conducted and the results are shown in FIG. 7. We noticed that the effect of the voltage also influenced the photodegradation efficiency of ZnO which increases with increased in applied voltage.

For comparison, photodegradation of MB using catalyst produced in a mixture of NaOH and (NH₄)₂SO₄, NaOH alone and (NH₄)₂SO₄ alone at 10 V via anodization method were conducted. The results are presented in FIG. 8. The results showed the ZnO catalyst produced in NaOH solution without ammonium sulfate was more effective. Zhang et al. [1] have reported the effect of anions such as SO₄ ²⁻, Cl⁻ and NO₃ ⁻ on the photocatalytic activity. The activity was reduced due to inhibitive effect of SO₄ ⁻ ions, which can be attributed to the competitive adsorption between SO₄ ²⁻ and methyl orange at the photocatalyst surface.

Nanoflakes of ZnO have been synthesized by the anodization of Zn foil in a mixture of ammonium sulfate and sodium hydroxide solution. Increasing the concentration of sodium hydroxide in the solution mixture leads to the formation of ZnO of different morphology. The presence of SO₄ ²⁻ anions stimulates active dissolution of Zn foil while higher concentration of sodium hydroxide lead to higher formation of nanoflakes zinc oxide. Furthermore, under constant stirring condition, more uniform nanoflakes ZnO was formed. Photocatalytic activity of the zinc oxide was evaluated by measuring the photocatalytic degradation of methylene blue in water under the illumination of UV light. Photocatalytic ZnO prepared in higher concentration of NaOH was found to be 9.57%. However, the catalysts show low catalytic activity in the photodegradation of methylene blue when prepared in the presence of ammonium sulfate and this is attributed to the inhibitive effect of SO₄ ²⁻ ions as well as competitive adsorption between SO₄ ²⁻ ions of and MB. 

1. An anodizing method for manufacturing zinc oxide nano flakes comprising: adding to a cell an anode electrode comprising a zinc film; adding to the cell a platinum wire as cathode electrode and separated from the anode electrode by a distance of 2-4 cm; adding an electrolyte to the cell, the electrolyte comprising a 0.0125 to 0.0375 molar solution of sodium hydroxide and 0.05 M solution of ammonium sulfate; passing a current of 5 to 15 volts direct current to convert the zinc anode to zinc oxide for a period of 50 to 100 minutes; and, removing and washing the anode as a source of nano flakes of zinc.
 2. The anodizing method of claim 1, wherein the zinc film is 0.2 mm thick.
 3. The anodizing method of claim 1, wherein the surface area of the film is 6.0 cm².
 4. The anodizing method of claim 1, wherein the distance between the anode and the cathode electrodes is 3 cm.
 5. The anodizing method of claim 1, wherein the temperature of the cell is between 20° C. and 30° C.
 6. The anodizing method of claim 1, wherein the current is passed for a period of 90 minutes.
 7. The anodizing method of claim 1, wherein the nano flakes have a leaf-shaped structure. 